A hydraulic power steering system for a vehicle includes a steering control valve. The control valve controls the pressure of hydraulic fluid flowing between a pump, a reservoir, and a hydraulic power steering actuator. A steering linkage connects the actuator to a pair of steerable vehicle wheels. When the driver of the vehicle rotates the steering wheel, the control valve responds by varying hydraulic fluid pressures in the actuator. The system thus provides hydraulic power steering assistance to the steerable wheels that are linked to the actuator.
The pump is driven by the vehicle engine. If the engine fails, the pump may fail to provide a flow of hydraulic fluid that is sufficient to operate the actuator. Therefore, a hydraulic power steering system may include an auxiliary pump which is driven by a ground-engaging vehicle wheel or other driving means. If the engine should fail while the vehicle is in motion, the auxiliary pump can provide a flow of hydraulic fluid sufficient for operating the actuator. Such a steering system further includes a switching valve between the control valve and the two pumps. The switching valve directs hydraulic fluid to the control valve selectively from the two pumps.